Cement is used to seal tubulars in boreholes. The cement is pumped through a one way valve at the lower end of the string to be sealed that is also known as a shoe. The pumped cement needs to be displaced from the tubular to the surrounding annulus after it is delivered from the surface. Different wiper plug systems have been devised to push the cement ahead of the plug until the plug is bumped on a landing shoulder in the vicinity of the shoe.
Liner wiper plugs are typically suspended at the top of a liner to be cemented with an open passage through the wiper plug thorough which the cement is delivered. A dart is then landed in the wiper plug and the two travel together to wipe the liner free of cement until the plug is bumped. The plug can have extending fins in parallel rows or it can be a solid block. A one or two plug system can be used and in each case a dart lands in the plug to move the two in tandem. Composite materials have been employed in such plugs to speed up milling that occurs after the plug or plugs are bumped and the cement sets. The normal procedure is to drill out the plug or plugs and the shoe and either extend the well or complete the well.
Because the plug or plugs are initially located in the liner or casing to be cemented, they are already of the appropriate size for the wiping task that needs to be done when they are deployed. However, issues can develop if the wiper is to be delivered through a smaller running string for the liner or casing to be cemented and then still be expected to wipe the inside dimension of the far larger casing or liner. One approach to addressing this problem has been the development of plugs made of compressible foam that can be squeezed into the running string and pushed with pressure to the casing or liner where the expectation is for the foam to then relax and retain its initially larger dimension. While introducing the plug to the larger diameter tube will allow it to regain its former shape, the problem with such foam plugs under differential pressure loading will be that the pressure will again deform the plug by compression to open bypass flow paths around it and thus undermining its ability to serve as an effective wiper plug. The rationale for such plugs is that they can pass restrictions on the way down and in theory still function effectively as a wiper plug after traversing a limited number of obstructions and reforming. Both foam darts and balls made of open cell rubber have been offered by Halliburton with the caveat that they cannot be used in cementing service where there is a series of tight restrictions.
Various attempts have been made to design wipers regardless of shape that can go through an obstruction and then continue to operate, generally in a tubular having the same drift above and below the obstruction. Some examples of such devices can be seen in US Publication 2008/0190613; U.S. Pat. No. 7,673,688 using a foam body and an external screening material; U.S. Pat. No. 5,435,386 showing a cement plug with a rubber mandrel and a foam exterior layer; U.S. Pat. No. 7,096,949 shows a wiper plug with an articulated seal that is actuated with applied differential pressure as illustrated in FIGS. 8a and 8b. US Publication 2010/0038086 teaches the use of stimulus responsive materials that can swell or get smaller in a production system to regulate the location of the produced flow into the wellbore using the stimuli that make plugs change dimension.
The present invention seeks to provide a plug that can be delivered in a smaller string to wipe effectively in a larger string. This is accomplished with shape memory foam that is brought past its transition temperature downhole so that it can pass through a running string and then revert to a larger original shape for effective wiping of the string being cemented or undergoing other downhole operations. The stimulus can be using well fluids or applied heat or reactive materials that are held apart for run in and then allowed to contact for an exothermic reaction that triggers the wiper to revert to the larger size suitable for wiping the larger tubular. Those skilled in the art will more fully appreciate the various aspects of the invention from a review of the description of the preferred embodiment and the associated drawings while appreciating that the full scope of the invention is to be determined by the appended claims.